The present invention relates to relieving stress in the microstructure of metal by induction heating, and more particularly to the stress-relief of welds and surrounding areas of base metal in low alloying steels employed in hostile subsurface environments such as are encountered in the petroleum industry.
In many areas of the world, hydrogen sulfide, commonly called "sour gas" is encountered where a petroleum well is drilled. The presence of sour gas in a well dictates a choice of materials which will not be subject to sulfide stress cracking, which may be described as the brittle failure of a metal under the combined action of tensile stress and corrosion in the presence of hydrogen sulfide in an aqueous environment.
The National Association of Corrosion Engineers (NACE) has published a Material Requirement dealing with "Sulfide Stress Cracking Resistant Metallic Material For Oil Field Equipment," NACE Standard MR-01-75 (1980 Revision), which standard is employed as a materials guideline by the petroleum industry when dealing with sour gas. The standard sets forth various requirements for the acceptable performance in sour gas of different ferrous and non-ferrous metals, fabrication, bolting, platings and coatings thereof, as well as requirements for various specific components and devices used in well drilling, testing, production and servicing. Low alloy steels, being those containing less than about five percent (5%) total alloying elements, are acceptable materials for a sour gas environment provided they meet certain requirements set forth in NACE Standard MR-01-75.
In general, if these steels contain less than one percent (1%) nickel (Ni), they are acceptable provided they are subjected to certain thermal treatments to alter the microstructure of the steel. It is emphasized by the NACE Standard that there is a definite correlation between sulfide stress cracking, heat treatment and hardness in metals, which correlation has been proven by extensive laboratory and field data. Hardness is in part a function of the stress present in the microstructure; the resistance of the metal to sulfide stress cracking is enhanced by the lowering of hardness by stress relief in the microstructure through heat treatment. As hardness is an accurate, nondestructively generated test parameter, it is extensively used to monitor materials performance. The Rockwell "C" Hardness Scale (HRC) as used in the NACE Standard is the primary basis for determination of an acceptable hardness, for sour gas equipment, although it should be understood that other hardness scales may be employed using suitable conversion factors for correlation purposes. As a rule, a hardness of HRC 22 is the maximum hardness allowable by NACE for low-alloy steels and welds thereon in a sour gas environment. A hardness of up to HRC 26 may be tolerated for certain tubular goods, but only if adequate performance is verified with a sulfide stress cracking test, a procedure necessitating additional expense. Therefore, it is desirable to obtain a hardness of HRC 22, it being understood that this figure may be the average of several tests, as long as the maximum HRC of a specimen does not exceed 23 or 24.
In many instances, a hardness of HRC 22 maximum for steel and welds thereon can be obtained by heat treating the material or apparatus in question in a furnace, but in other instances this is impractical, such as where the apparatus is assembled and then welded, the assembly including components such as elastomeric seals, which are destroyed by the high furnace temperatures. Likewise, even in the absence of seals, there may be finished surfaces in the apparatus which would be damaged by prolonged exposure to high temperatures necessitated by furnace treatment. Thus, there presently exists a problem in the production of some material or assembly of apparatus to the NACE Standard.